38 PLANT PHYSIOLOGY 



only by changing the demand for energy. To repair an 

 injury or to close a wound new cell-walls must be deposited, 

 new cells must be formed, more food must be manufactured, 

 or stored food must be dissolved, the food must be carried 

 to the seat of the injury and used there, and other kinds of 

 work must be done. For all this extra work extra energy 

 must be supplied, and hence the rate of respiration must be 

 increased. This increased rate shows itself in plants as in 

 animals by the increased production of carbon-dioxide and 

 by a rise in temperature (wound-fever). In parts and in or- 

 ganisms most rapidly growing the rate of respiration will be 

 high, but growth is only one function and when it decreases 

 other functions may become more active, keeping the de- 

 mand for energy uniform, and hence the rate of respiration 

 will not necessarily decrease with a decreasing growth- 

 rate. 



The rate of respiration varies with the total demand for 

 energy, not with any one function. For instance, the rate 

 of respiration and consequently the liberation of energy 

 (heat) reach their maximum in the spathes of Aroids* after 

 the parts have passed the period of most rapid growth ; for, 

 though the growth-rate diminishes, the various processes 

 concerned in and stimulated by fertilization demand an 

 equal or even greater amount of energy. 



In normal respiration the volumes of oxygen fixed and 

 of carbon-dioxide evolved are approximately equal. When 

 this ratio is not obtained in an experiment, the difference is 

 to be accounted for in one or more of the following ways. 

 First, the ratio will vary with the amount of oxygen en- 

 closed in the body of the organism at the beginning of the 

 experiment. This oxygen will be used before more can be 

 absorbed. Second, only so much of the carbon-dioxide 

 evolved (exhaled) can be measured as is not absorbed, or 

 does not remain enclosed, by the tissues, or, in the case of 

 water-plants, is not dissolved in the water. Third, other 

 substances than carbon-dioxide and water may be formed 

 in respiration ( e. g. organic acids, alcohol, etc. ) and there- 

 fore the different amounts of these other products in differ- 



* See Pfeffer, Pflanzenphysiologie, Bd. 11., p. 404, of the first edition. 



